Method of making a thermoplastic lens coated with a thermosetting protective layer

ABSTRACT

A method for making a thermoplastic optical lens having at least one side coated with a thermosetting protective film is provided. The method includes steps of applying a thermosetting coating composition to a molding surface of the molding shell, disposing the thermoplastic lens blank in a horizontal position and vertically above the molding shell, heating the lens blank to cause thermal sagging under low static load conditions as the lens blank comes into contact with the coating composition on the molding surface and continuing heating until the lens blank mates with the molding surface.

FIELD OF THE INVENTION

The present invention is generally concerned with optical lenses, and inparticular ophthalmic lenses, made from a thermoplastics syntheticmaterial such as polycarbonate, for example.

It is more particularly directed to the situation in which, as in themethod which is the subject matter of French patent application number88 14046 filed 27 Oct. 1988 (published number 2 638 391), a lens of thiskind is formed from a lens blank which is heated so that natural thermalsagging under a reduced static load causes it to mate with the shape ofan underlying molding shell.

BACKGROUND OF THE INVENTION

An advantage of a shaping process of this kind is that it can producedirectly the final geometry of the required optical lens so that thelatter is directly usable by the optician.

Another and more important advantage is that it produces only weak andeven tension stresses within the optical lens.

In practise these internal tension stresses are only in the order of onetenth of those in an optical lens made in the conventional manner byinjection molding.

However, regardless of the process by which they are shaped,thermoplastics synthetic material optical lenses have a furtherdisadvantage in their mediocre resistance to abrasion.

It has therefore been proposed to coat at least one side of such lenseswith a thermosetting synthetic material protection film which has goodresistance to abrasion.

Various coating methods have been proposed, the diversity of which bearswitness to the difficulty of this problem.

The most usual method is first to shape the optical lens to its finalgeometry, whether by injection molding or by natural thermal sagging,and then to apply the necessary thermosetting synthetic material coatingcomposition or varnish to the optical lens, and finally to harden, i.e.polymerize the coating composition by heating or by radiation.

Apart from the fact that the thickness of the resulting film is notalways easy to control, a drawback of this coating method is that by itsvery nature it introduces an additional stage of manufacture,necessarily being carried out at a different stage from that in whichthe optical lens itself is made.

To combine these two stages into one attempts have been made to form theoptical lens to its final geometry when already coated with itsprotective film, the latter being at least partially hardenedbeforehand.

This is the case, for example, in French patent application number 7720742 filed 6 Jul. 1977 (published number 2 358 256) and in U.S. Pat.Nos. 2,322,310, 2,481,809 and 2,640,227.

All these methods conventionally require the application of highpressures (in the order of 7 kg/cm² to 300 kg/cm²) in a press and in anuncontrolled manner and therefore result in non-negligible internaltension stresses in the optical lens finally obtained.

When the protection film applied to the optical lens is totallypolymerized before final shaping of the lens, the shaping almostinevitably causes cracking of the protection film because of thestresses occurring at the interface between it and the substrate itcovers.

If the protection film is only partially polymerized before finalshaping of the optical lens it is difficult to control its thickness atthe end of the shaping process.

Finally, if the pressure that has to be used to shape the optical lensto its final shape is particularly high, it necessitates softening ofthe substrate beforehand, which is undesirable.

OBJECT AND SUMMARY OF THE INVENTION

A general object of the present invention is a method for obtaining in asingle stage but without the above drawbacks a thermoplastics syntheticmaterial optical lens coated on at least one side with a thermosettingsynthetic material protection film.

It is based on the observation, not previously made, that if in theshaping process by natural thermal sagging a mold release agent is usedon the molding shell, the mold release agent is transferred from themolding shell to the optical lens, adhering thereto and, surprisingly,that the same applies to any other coating composition if the latter isnot fully polymerized.

Based on this observation, the method according to the invention isgenerally characterized in that starting from a thermoplastics syntheticmaterial lens blank and a molding shell to the molding surface of whichof a thermosetting synthetic material coating composition has beenapplied, said lens blank is disposed horizontally, vertically above saidmolding shell and heated to cause natural thermal sagging under areduced static load of the lens blank until by contact with the coatingcomposition on the molding surface of the molding shell it mates withthe shape of said molding surface.

At the end of this process the coating composition, used on its own ortogether with a mold release agent, is entirely transferred from themolding shell to the lens blank, which is shaped to its final shape, thecoating adhering to the lens in the required manner.

Thus in a single stage there is easily obtained an optical lens madefrom a thermoplastics synthetic material by natural thermal sagginghaving only weak and even internal stresses or no internal stresses atall and directly usable by an optician because it is directly shaped toits final shape and is coated on at least one side with thethermosetting synthetic material protection film having all thenecessary abrasion resistance and adhering perfectly to the substrate itcovers.

The technique usually called "in mold coating" (IMC) is a known methodof coating an article within the mold in which it is made.

However, until now this technique has been used only for thermosettingsynthetic material substrates of the same kind as the coatingcomposition applied to them and the latter is introduced into the moldin the liquid state.

The combination is reactive, interaction inevitably occurring betweenthe substrate during polymerization and the liquid coating compositionwith which it is in contact. The end result is some homogenization ofthis combination, to the detriment of the inherent qualities of thecoating composition and therefore those of the protection film itproduces.

This is absolutely not the case in the method according to the inventionwhich, to the contrary, is intended to produce and succeeds in producinga heterogeneous product with a clear distinction but nevertheless allthe required adherence at the interface between a thermoplasticssynthetic material substrate and a thermosetting synthetic materialprotection film retaining intrinsically all its hardening quality.

It has to be emphasized that this result is entirely surprising.

The treated lens blank comes into contact progressively with the coatingcomposition as it sags when heated at the same time as the coatingcomposition is polymerized, there having previously been nothing tosuggest that, despite the changing nature of the coating composition,when its application to the lens blank itself being shaped is completeda perfectly satisfactory protection film could be obtained.

To the contrary, it has been necessary to overcome some prejudice whichholds that it is not normally possible to obtain a satisfactory productreproducibly if the nature of the constituents of the product changesduring its formation.

This is the case in the method according to the invention, however.

The invention also reconciles in a satisfactory manner two normallycontradictory requirements, yielding good adherence between twoconstituents of different kinds while minimizing the risk of tensionstresses at the interface between them.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention emerge from the followingdescription given by way of example with reference to the appendeddiagrammatic drawings in which:

FIG. 1 is a perspective view of an optical lens which can be formed bythe method according to the invention;

FIG. 2 is a partial view of the lens to a larger scale and in axialcross-section on the line II--II in FIG. 1;

FIG. 3 shows to a still larger scale part of FIG. 2 marked by a frameIII in FIG. 2;

FIG. 4 is an axial cross-section view to a different scale showing theimplementation of the method according to the invention prior to shapingof the original lens blank;

FIG. 5 shows to a larger scale the part of FIG. 4 shown by a frame V inFIG. 4;

FIGS. 6A, 6B are axial cross-section views analogous to those in FIG. 4for two successive phases in shaping of the optical lens;

FIG. 7 is a diagrammatic elevation view to a different scale of an ovenused for the shaping method.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the figures, the overall aim is to obtain an optical lens10 which, forming a thermoplastics synthetic material (for examplepolycarbonate) substrate, is coated on at least one side with athermosetting synthetic material protection film 11.

In the embodiment shown in FIGS. 1 to 3 the optical lens 10 is aconcave-convex ophthalmic lens and before it is trimmed to size, asshown, it has a circular contour with a flat edge of diameter D andaverage thickness E.

In this embodiment only its convex side 12 is coated with a protectionfilm 11.

The thickness of the protection film 11 is e.

This thickness e is only a fraction of the thickness E, being inpractise less than one thousandth of the latter.

The optical lens 10 is made by the natural thermal sagging method asdescribed in French patent no 2 638 391.

The content of this French patent is hereby incorporated by way ofreference.

Briefly, referring to FIG. 4, starting from a thermoplastics syntheticmaterial (for example polycarbonate) lens blank 10' and a molding shell14I having a molding surface 15I whose geometry corresponds to thatrequired for the corresponding side of the lens to be formed, the lensblank 10' is disposed horizontally, vertically above the molding shell14I with the molding surface 15I of the molding shell 14I facingupwards, and heated to cause natural thermal sagging under a reducedstatic load of the lens blank 10' until it mates with the shape of themolding surface 15I.

In the embodiment shown, and in the method described in detail in Frenchpatent no 2 638 391, the molding shell 14I rests on a shoulder 16projecting from a lower part of the inside surface of a ring 17, thelens blank 10' itself resting on a shoulder 18 projecting from theinside surface of a ring 19 which bears on the molding shell 14I andwhich has at its base vents 20. A molding shell 14S is mounted to slidefreely in the ring 17 above the lens blank 10', bearing on the latter,its downwardly facing molding surface 15S having the required geometryof the respective side of the lens 10 to be shaped.

The ring 17 defines with the molding shells 14I, 14S a mold 22 which inthe embodiment shown is thermally insulated on the outside and over allof its height by an insulative thermal jacket 23.

As an alternative to this the ring 17 and the insulative thermal jacket23 can be in one piece.

To bring about the required thermal sagging the mold 22 and the lensblank 10' therein are inserted in the direction of the arrow F1 in FIG.7 into a tunnel oven 24 which has, in addition to a loading area 25 atits inlet end and an offloading area 26 at its outlet end, a pluralityof successive separate areas Z1, Z2, . . . ZN with their temperatureindividually controlled and a conveyor 28 running along its entirelength for transferring the mold 22 from its entry end to its exit endin the direction of the arrow F2 in FIG. 7.

As an alternative to this the tunnel oven 24 can have only one heatingarea, the temperature in which is varied during the thermal saggingprocess.

After passing through the tunnel oven 24 the mold 22 is removedtherefrom in the direction of the arrow F3 in FIG. 7.

In the embodiment shown the lens blank 10' is flat.

The side of the optical lens 10 to be shaped which is to be coated witha protection film 11 being its convex side 12, the molding surface 15Iof the bottom molding shell 14I is concave and the molding surface 15Sof the top molding shell 14S is convex.

According to the invention, and prior to the natural thermal saggingprocess, a thermosetting synthetic material coating composition 11' isapplied to the molding surface 15I of the molding shell 14I in the formof a film covering all of this molding surface 15I.

The lens blank 10' therefore comes into contact with this coatingcomposition 11' when, as a result of natural thermal sagging, it mateswith the shape of the molding surface 15I of the molding shell 14Idisposed beneath it.

At the end of the natural thermal sagging process, and as shown in FIG.6B, the coating composition 11' is transferred from the molding shell14I to the lens blank 10', the coating composition 11' then forming onthe optical lens 10 obtained the required protection film 11, adheringas required to its convex side 12.

On first contact of the lens blank 10' with the coating composition 11',the latter has preferably reached a temperature in excess of its gelpoint in order to limit if not eliminate any lateral flow thereof.

In other words, the coating composition 11' is preferably no longerliquid at this time.

To this end, and as described in more detail below, it is first dried orprepolymerized.

In any event, it preferably has added to it a catalyst adapted to renderpolymerization progressive and so favor its adherence to the lens blank10'.

As shown diagrammatically in FIG. 6A first contact of the lens blank 10'with the coating composition 11' is preferably in the central area ofthe molding shell 14I.

This prevents any air pocket being trapped between the molding shell 14Iand the lens blank 10' during its natural thermal sagging.

One function of the ring 19 is to achieve this result.

As it passes through the tunnel oven 24 inside the mold 22 the lensblank 10' is heated to a temperature T between its vitreous transitiontemperature T_(g) and its melting point T_(f), without reaching thelatter.

The coating composition 11' used must naturally be chosen accordingly.

In other words, it must be able to withstand the temperature T withoutdeteriorating.

It must also be chosen such that its shrinkage between this temperatureT and room temperature is comparable with that of the thermoplasticssynthetic material of the lens blank 10'.

This is so that the protection film 11 that it forms on the surface ofthe resulting optical lens 10 is not detached from the latter oncooling.

To the contrary, it seems that although there is a clear interfacebetween the protection film 11 and the substrate that it covers, thereis excellent adherence between it and the latter, passing the usualadherence tests, and considered to be the result of a physical-chemicalprocess.

Release of the optical lens 10 from the mold is also good, itsprotection film 11 not adhering to the molding shell 14I.

The static load to which the lens blank 10' is subjected during itsnatural thermal sagging process is preferably restricted to a valuebelow 10 kg.

This static load is preferably restricted to a value between 2.5 kg and50 g.

As shown, this static load in practise is the result of gravity alone.

It is restricted to the weight of the top molding shell 14S, which canhave an additional weight 30 placed on the top, as showndiagrammatically in dashed outline in FIG. 4.

The method according to the invention can be implemented as follows.

First the bottom molding shell 14I is varnished, i.e. the coatingcomposition 11' used is applied to its molding surface 15I.

This varnish can be applied in the usual way by dipping into a bath orby centrifuging.

These techniques are well known in themselves and need not be describedin more detail here.

Depending on the configuration of the lens blank 10', the varnishedmolding shell 14I is:

1) either used directly without preliminary treatment,

2) or, especially if the lens blank 10 is precurved:

a) allowed to remain at room temperature for sufficient time for thecoating composition 11' on it to be dust-dry, or

b) heated to a temperature above the gel point of the coatingcomposition 11'.

The dried or preheated bottom molding shell 14I is then placed in thering 17 followed by the ring 19, the lens blank 10' and the top moldingshell 14S, with the additional weight 30 if required.

If the lens blank 10' is flat it is at a distance from the molding shell14I.

If it is precurved it bears against the latter.

Either way, the mold 22 is then placed in the tunnel oven 24.

As it passes through the latter the temperature is increased topolymerize further the coating composition 1' on the bottom moldingshell 14I and to initiate thermal sagging of the lens blank 10'.

If the lens blank 10' is flat, as shown here, it therefore comes intocontact with the coating composition 11' in the central area of themolding shell 14I, after about 30 minutes (FIG. 6A).

As the temperature of the coating composition 11' is above its gelpoint, as mentioned above, there is no significant flow of the coatingcomposition 11' towards the edge of the molding shell 14I.

As thermal sagging of the lens blank 10' continues, the lens blank 10'progressively mates with the shape of the molding surface 15I of themolding shell 14I (FIG. 6B) and at the same time polymerization of theprotection composition 11' on the latter continues.

To illustrate the invention more clearly there follows an example of itsuse for a flat lens blank 10' having a diameter D in the order of 80 mm.

I. Preparation of coating composition 11'

The thermosetting coating composition is preferably a polysiloxane typevarnish.

361.08 g of glycidoxypropyltrimethoxysilane having formula RSi(OR')₃(where R=glycidoxypropyl and R'=CH₃) was hydrolized in 82.89 g of 0.1Nhydrochloric acid.

Hydrolysis continued for 24 hours.

To increase abrasion resistance, 94.55 g of a solution of colloidalsilica in methanol was added to the hydrolysate obtained, the colloidalsilica content of the solution being 30% and the silica particlediameter being 13 mμm (millimicrons).

1.19 g of aluminium acetylacetonate were then added as catalyst and66.03 g of ethylcellosolve and 171.55 g of methanol were added assolvent.

0.6 g of FC430 were then added as surfactant.

After complete polymerization the colloidal silica content of the dryextract was in the order of 10% and the RSi O_(3/2) content was in theorder of 90%.

II. Treatment of molding shells 14I, 14S

The molding shells 14I, 14S, the molding surfaces 15I, 15S of which canhave any geometry, for example progressive toroidal or cylindricalgeometry, can be of mineral glass subjected to thermal annealing, forexample.

Their molding surfaces 15I, 15S were coated with a mold release agentsuch as dimethyldichlorosilane either by vapor deposition or byapplication of a mixture in isopropyl alcohol containing 2% by weight ofthis mold release agent.

Alternatively, the molding shells 14I, 14S can be used as they are.

In this case, however, their molding surfaces 15I, 15S are preferablywiped with acetone to degrease them and render them chemically clean.

III. Application of coating composition 11' to molding shell 14I

This application is done by immersion in a bath, for example.

The bath was held at a temperature of 3° C. to 4° C., the molding shell14I introduced into it vertically, held in it for 1 minute 25 secondsand removed from it with a dewetting time (i.e. a raising time) of oneminute.

Alternatively, however, and as previously mentioned, the coatingcomposition 11' can be applied to the molding shell 14I by centrifuging.

In either case, if the starting lens blank 10' is flat, as here, themolding shell 14I entirely coated, and therefore having a moldingsurface 15I coated, with a film of coating composition 11', was dried atroom temperature until it was dust-dry, i.e. until it had polymerized sothat it was no longer tacky.

IV. Heat treatment

The static load applied to the lens blank 10' in the mold 22 into whichit had been placed was limited to the weight of the molding shell 14S ontop of it, which was 100 g.

The mold 22 was placed directly in the tunnel oven 24, the initialtemperature in which was 110° C.

There it underwent the following cycle:

temperature increased from 110° C. to 160° C. in 30 minutes,

temperature maintained at 160° C. for 30 minutes,

temperature increased from 160° C. to 196° C. in 35 minutes,

temperature maintained at 196° C. for 50 minutes,

temperature increased from 196° C. to 203° C. in ten minutes,

temperature reduced from 203° C. to 30° C. in two hours.

V. Characteristics of resulting protection film 11

Thickness: 2.56 μm.

Adherence tests:

A first adherence test was carried out according to French standardAFNOR 76 FNT 30-038, in which results are classified as degree 0 throughdegree 5.

It entails cutting the protection film 11 into a cross-hatched mesh ofincised lines, applying adhesive tape to the cross-hatched protectionlayer 11 and attempting to pull it off using the tape.

Result: degree 0.

The edges of the cuts remained perfectly smooth and none of the squaresthereby was detached.

As a control, a second adherence test of the same type was carried outon an optical lens 10 provided with a protection film 11 and previouslyimmersed in boiling water for 30 minutes.

The results were the same.

Abrasion resistance tests:

BAYER test:

This yielded a value of 3 to 4, substantially the same as that ofprotection layers 11 obtained by a prior art method.

Steel wool test:

This also yielded a result comparable to that for protection films 11obtained in the usual way.

Of course, the present invention is not limited to the embodimentspecifically described, but encompasses any variant execution thereof.

In particular, the silica content of the colloidal silica suspensionused in the coating composition for favoring its abrasion resistance,given as a percentage dry extract relative to the final product, canvary between 0% and 30%, preferably between 10% and 30%.

It is most preferably below 30%, however.

Above 30% cracking of the protection layer can occur, shrinkage of thislayer being then too great.

Similarly, the quantity of catalyst used in the composition to renderpolymerization progressive can be varied.

However, it is usually between 0% and 0.5% by weight, representing acompromise between good abrasion resistance and good adherence.

At 0% adherence is satisfactory but abrasion resistance can be mediocreand beyond 0.5% polymerization is usually too fast for adherence to besatisfactory.

Between 0% and 0.3% by weight of the catalyst is preferably used in thecoating composition.

Applications of the invention are naturally not restricted to thetreatment of concave-convex ophthalmic lenses, but extend more generallyto the treatment of any optical lens, regardless of its profile.

Finally, it goes without saying that both sides of an optical lens canbe coated in this way with a protection film.

I claim:
 1. Method for making a thermoplastic optical lens having atleast one side coated with a thermosetting protective film comprisingthe steps of: providing a polymerized and hardened thermoplastic lensblank and a molding shell, applying a thermosetting coating compositionto a molding surface of the molding shell, disposing the lens blank in ahorizontal position and vertically above the molding shell, and heatingthe lens blank to cause thermal sagging under low static load conditionsas the lens blank comes into contact with the coating composition on themolding surface and continuing heating until the lens blank mates withthe molding surface.
 2. Method according to claim 1, wherein the coatingcomposition is at a temperature above its gel point when the lens blankfirst comes into contact therewith.
 3. Method according to claim 1,wherein the coating includes a catalyst for progressive polymerization.4. Method according to claim 1, wherein thermal sagging of the lensblank is effected such that the lens blank first comes into contact withthe coating composition at a central area of the molding surface. 5.Method according to claim 1, wherein the static load exerted against thelens blank is less than 10 kg.
 6. Method according to claim 1, whereinthe static load exerted against the lens blank is between about 2.5 kgand about 50 g.
 7. Method according to claim 6, wherein the lens blankhas a diameter of about 80 mm and the static load exerted on the lensblank is about 100 g.
 8. Method according to claim 1, wherein after thecoating composition is applied to the molding surface, the molding shellis heated to a temperature above the gel point of the coatingcomposition.
 9. Method according to claim 1, wherein after the coatingcomposition is applied to the molding surface, the molding shell ismaintained at room temperature for sufficient time for the coatingcomposition to become dust-dry.
 10. Method according to claim 1, furthercomprising freely slidably mounting a complementary molding shell in aring above the lens blank, the force of gravity on the complementarymolding shell defining the static load exerted on the lens blank. 11.Method according to claim 1, further comprising freely slidably mountinga complementary molding shell in a ring above the lens blank, applyingan additional weight to the complementary molding, the force of gravityon the complementary molding shell and additional weight defining thestatic load exerted on the lens blank.
 12. Method according to claim 1,wherein the coating composition is a polysiloxane varnish.